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Igneous Rocks/EPCC/LM4

3.
What is a rock?What is a rock?
 A rock (is most often)is hard andA rock (is most often)is hard and
made up of minerals.made up of minerals.
 Why most often?Why most often?
What is coal?What is coal?
What is obsidian?What is obsidian?
Rock Gypsum?Rock Gypsum?
 The are criteria has exceptions!The are criteria has exceptions!

10.
What Is an Igneous Rock?What Is an Igneous Rock?
Igneous rocks vary greatly.Igneous rocks vary greatly.
– Some contain large mineral grains.Some contain large mineral grains.
– Others contain grains so small they can barely beOthers contain grains so small they can barely be
seen under a high power microscope.seen under a high power microscope.
– Igneous rocks also vary greatly in color.Igneous rocks also vary greatly in color.
 All igneous rocks are formed through theAll igneous rocks are formed through the
cooling and solidification of magma/lava.cooling and solidification of magma/lava.

13.
Intrusive Versus Extrusive IgneousIntrusive Versus Extrusive Igneous
RocksRocks
 Extrusive igneous rocks form when magmaExtrusive igneous rocks form when magma
cools on Earth’s surface, where they havecools on Earth’s surface, where they have
been “extruded.”been “extruded.”

15.
Textures in Igneous RocksTextures in Igneous Rocks
Texture – size, shape and orientation of theTexture – size, shape and orientation of the
crystals within a rockcrystals within a rock
Sizes of mineral grains:Sizes of mineral grains:
– Intrusive rocks are coarse-grained. (Phaneritic)Intrusive rocks are coarse-grained. (Phaneritic)
 Magma that solidifies in the crust cools slowly and hasMagma that solidifies in the crust cools slowly and has
sufficient time to form large mineral grains.sufficient time to form large mineral grains.

17.
Textures in Igneous RocksTextures in Igneous Rocks
– Extrusive rocks are fine-grained.Extrusive rocks are fine-grained.
Lava that solidifies on the surface usuallyLava that solidifies on the surface usually
cools rapidly, allowing insufficient timecools rapidly, allowing insufficient time
for large crystals to grow.for large crystals to grow.
– Fine-grained igneous rock is called anFine-grained igneous rock is called an
aphaniticaphanitic

19.
Textures in Igneous RocksTextures in Igneous Rocks
– AA porphyryporphyry is an igneous rock in which 50% or moreis an igneous rock in which 50% or more
of the rock is coarse mineral grains scattered through aof the rock is coarse mineral grains scattered through a
mixture of fine mineral grains.mixture of fine mineral grains.
– Porphyritic texturePorphyritic texture
– The isolated large grains areThe isolated large grains are phenocrystsphenocrysts..

23.
Textures in Igneous RocksTextures in Igneous Rocks
– Another common variety of igneous rock is pumice,Another common variety of igneous rock is pumice,
a mass of glassy bubbles of volcanic origin.a mass of glassy bubbles of volcanic origin.
– Texture is vesicular – the bubbles have “popped”Texture is vesicular – the bubbles have “popped”
and left a vesicleand left a vesicle
– Volcanic ash is also mostly glassy because theVolcanic ash is also mostly glassy because the
fragments of magma cooled too quickly tofragments of magma cooled too quickly to
crystallizecrystallize..

27.
Solidification of MagmaSolidification of Magma
 A magma of a given composition canA magma of a given composition can
crystallize into many different kinds ofcrystallize into many different kinds of
igneous rock.igneous rock.
 Solidifying magma forms several differentSolidifying magma forms several different
minerals which start to crystallize from theminerals which start to crystallize from the
cooling magma at different temperaturescooling magma at different temperatures..

28.
Solidification of MagmaSolidification of Magma
 Crystal-melt separation can occur in aCrystal-melt separation can occur in a
number of ways:number of ways:
– Compression can squeeze melt out of a crystal-meltCompression can squeeze melt out of a crystal-melt
mixture.mixture.
– Dense, early crystallized minerals may sink to theDense, early crystallized minerals may sink to the
bottom of a magma chamber, thereby forming a solidbottom of a magma chamber, thereby forming a solid
mineral layer covered by melt.mineral layer covered by melt.
 However a separation occurs, theHowever a separation occurs, the
compositional changes it causes are calledcompositional changes it causes are called
magmatic differentiation by fractionalmagmatic differentiation by fractional
crystallizationcrystallization..

29.
Bowen’s Reaction SeriesBowen’s Reaction Series
 Canadian-born scientist N. L. BowenCanadian-born scientist N. L. Bowen
(1887-1956) first recognized the(1887-1956) first recognized the
importance ofimportance of magmatic differentiation bymagmatic differentiation by
fractional crystallization.fractional crystallization.
 Bowen argued that a single magma couldBowen argued that a single magma could
crystallize into both basalt and rhyolitecrystallize into both basalt and rhyolite
because of fractional crystallizationbecause of fractional crystallization..

30.
Bowen’s Reaction SeriesBowen’s Reaction Series
 Bowen knew that plagioclases thatBowen knew that plagioclases that
crystallize from basaltic magma are usuallycrystallize from basaltic magma are usually
calcium-rich (anorthitic).calcium-rich (anorthitic).
 Plagioclases formed from rhyolitic magmaPlagioclases formed from rhyolitic magma
are commonly sodium-rich (albitic).are commonly sodium-rich (albitic).
 Bowen called such a continuous reactionBowen called such a continuous reaction
between crystals and melts a continuousbetween crystals and melts a continuous
reaction series.reaction series.

31.
Bowen’s Reaction SeriesBowen’s Reaction Series
 Bowen identified several sequences ofBowen identified several sequences of
reactions besides the continuous reactionreactions besides the continuous reaction
series of the feldspars.series of the feldspars.
 When basalt cools down, one of theWhen basalt cools down, one of the
earliest minerals to form is olivine.earliest minerals to form is olivine.
– Olivine contains about 40 percent SiOOlivine contains about 40 percent SiO22 by weight.by weight.
– Basaltic magma contains 50 percent SiOBasaltic magma contains 50 percent SiO22..
 Crystallization of olivine will leave theCrystallization of olivine will leave the
residual liquid a little richer in silica.residual liquid a little richer in silica.

32.
Bowen’s Reaction SeriesBowen’s Reaction Series
 The solid olivine reacts with silica in the meltThe solid olivine reacts with silica in the melt
to form a more silica-rich mineral, pyroxene.to form a more silica-rich mineral, pyroxene.
 The pyroxene in turn can react to formThe pyroxene in turn can react to form
amphibole.amphibole.
 Amphibole can react to form biotite.Amphibole can react to form biotite.
 Such a series of reactions is called aSuch a series of reactions is called a
discontinuous reaction seriesdiscontinuous reaction series..

35.
Mineral AssemblagesMineral Assemblages
What it tells usWhat it tells us

36.
Mineral Assemblage In IgneousMineral Assemblage In Igneous
RocksRocks
 Once the texture of an igneous rock isOnce the texture of an igneous rock is
determined, its name will depend on itsdetermined, its name will depend on its
mineral assemblage. All common igneousmineral assemblage. All common igneous
rocks consist largely of:rocks consist largely of:
– Quartz.Quartz.
– Feldspar (both potassium feldspar and plagioclase).Feldspar (both potassium feldspar and plagioclase).
– Mica (both muscovite and biotite).Mica (both muscovite and biotite).
– Amphibole.Amphibole.
– Pyroxene.Pyroxene.
– Olivine.Olivine.

39.
Intrusive (Coarse-grained) IgneousIntrusive (Coarse-grained) Igneous
RocksRocks
 GraniteGranite is quartz-bearing rock in whichis quartz-bearing rock in which
potassium feldspar is at least 65 percent bypotassium feldspar is at least 65 percent by
volume of the total feldspar present.volume of the total feldspar present.

40.
Intrusive (Coarse-grained) IgneousIntrusive (Coarse-grained) Igneous
RocksRocks
 DioriteDiorite::
— The chief mineral in diorite is plagioclase.The chief mineral in diorite is plagioclase.
— Either or both amphibole and pyroxene are invariablyEither or both amphibole and pyroxene are invariably
present.present.

41.
Intrusive Igneous RocksIntrusive Igneous Rocks
 Dark-colored diorite grades intoDark-colored diorite grades into gabbro.gabbro.
— In gabbro, dark-colored minerals pyroxene and olivineIn gabbro, dark-colored minerals pyroxene and olivine
exceed 50 percent of the volume of the rockexceed 50 percent of the volume of the rock..
 A coarse-grained igneous rock in whichA coarse-grained igneous rock in which
olivine is the most abundant mineral isolivine is the most abundant mineral is
called a peridotite.called a peridotite.
 Gabbros and peridodites can be found inGabbros and peridodites can be found in
both the oceanic and the continental crust.both the oceanic and the continental crust.

43.
Extrusive Igneous RocksExtrusive Igneous Rocks
 Andesite:Andesite:
– Compositionally equal to a dioriteCompositionally equal to a diorite
– Named for the Andes.Named for the Andes.
 Basalt:Basalt:
– Compositionally equivalent to coarse-grainedCompositionally equivalent to coarse-grained
gabbro, fine-grained basalt is the most common kindgabbro, fine-grained basalt is the most common kind
of extrusive igneous rock.of extrusive igneous rock.
– The dominant rock of the oceanic crustThe dominant rock of the oceanic crust..

47.
Pyroclasts, Tephra, And TuffsPyroclasts, Tephra, And Tuffs
 A fragment of rock ejected during a volcanicA fragment of rock ejected during a volcanic
eruption is called aeruption is called a pyroclastpyroclast..
– Rocks formed from pyroclasts are pyroclasticRocks formed from pyroclasts are pyroclastic
rocksrocks..
 Geologists commonly refer to a deposit ofGeologists commonly refer to a deposit of
pyroclasts aspyroclasts as tephratephra, a Greek name for ash., a Greek name for ash.
– Tephra is a collective term for all airborne pyroclasts.Tephra is a collective term for all airborne pyroclasts.

48.
Pyroclasts, Tephra, And TuffsPyroclasts, Tephra, And Tuffs
 Tephra particles are categorized by size:Tephra particles are categorized by size:
– Bombs: greater than 64 mm in diameterBombs: greater than 64 mm in diameter
– Lapilli: between 2 and 64 mmLapilli: between 2 and 64 mm
– Ash: smaller than 2 mm.Ash: smaller than 2 mm.
 Tephra is igneous when it goes up butTephra is igneous when it goes up but
sedimentary when it comes down.sedimentary when it comes down.

49.
Pyroclasts, Tephra, And TuffsPyroclasts, Tephra, And Tuffs
 Pyroclastic rocks are transitional betweenPyroclastic rocks are transitional between
igneous and sedimentary rocks.igneous and sedimentary rocks.
 When bomb-sized tephra are transformed intoWhen bomb-sized tephra are transformed into
a rock they are calleda rock they are called agglomeratesagglomerates..
 They are calledThey are called tuffstuffs when particles are eitherwhen particles are either
lapilli or ash.lapilli or ash.

52.
PlutonsPlutons
 All bodies of intrusive igneous rock,All bodies of intrusive igneous rock,
regardless of shape or size, are calledregardless of shape or size, are called plutons,plutons,
after Pluto, the Greek god of the underworld.after Pluto, the Greek god of the underworld.
 Plutons are given special names depending onPlutons are given special names depending on
their shapes and sizes.their shapes and sizes.

54.
Minor Plutons: Dikes, Sills, andMinor Plutons: Dikes, Sills, and
LaccolithsLaccoliths
 AA dikedike is a tabular, sheet-like (thin butis a tabular, sheet-like (thin but
laterally extensive) body of igneous rock thatlaterally extensive) body of igneous rock that
cuts across the layering or fabric of the rockcuts across the layering or fabric of the rock
into which it intrudes.into which it intrudes.
 AA SillSill is tabular and sheet-like, like a dike,is tabular and sheet-like, like a dike,
but runs parallel to the layering or fabric ofbut runs parallel to the layering or fabric of
the rocks into which it intrudes.the rocks into which it intrudes.

56.
Minor Plutons: Dikes, Sills, andMinor Plutons: Dikes, Sills, and
Laccoliths (2)Laccoliths (2)
 AA laccolithlaccolith is parallel to the layering of theis parallel to the layering of the
rocks into which it intrudes, but forces therocks into which it intrudes, but forces the
layers of rock above it to bend, forming alayers of rock above it to bend, forming a
dome.dome.
 AA volcanic pipevolcanic pipe is the roughly cylindricalis the roughly cylindrical
conduit that once fed magma upward to aconduit that once fed magma upward to a
volcanic vent.volcanic vent.

59.
Major PlutonsMajor Plutons
 AA batholithbatholith is the largest kind of pluton. It isis the largest kind of pluton. It is
an intrusive igneous body of irregular shapean intrusive igneous body of irregular shape
that cuts across the layering or other fabric ofthat cuts across the layering or other fabric of
the rock into which it intrudes.the rock into which it intrudes.
– The largest batholith in North America,The largest batholith in North America,
approximately 1500 km long, is the Coast Rangeapproximately 1500 km long, is the Coast Range
batholith of British Columbia and southern Alaska.batholith of British Columbia and southern Alaska.
– The magma from which a batholith forms intrudesThe magma from which a batholith forms intrudes
upward from its source deep in the continental crust.upward from its source deep in the continental crust.